docs: def _get_contrast(self, con_id):
          """
          Retreive contrast with identifier 'con_id' from the list of contrast
          objects stored in self.contrasts
          """
          for contrasts in list(self.contrasts.values()):
              for contrast in contrasts:
                  if contrast.estimation.id == con_id:
                      return contrast
          raise Exception("Contrast activity with id: " + str(con_id) +
                          " not found.")